Various polymer microparticles are used as various spacers used in liquid crystals and the like, antiblocking agents for resin films, fillers for chromatography, ion exchange resins, plastic modifiers, and fluidity/smoothness improving agents for various powder bodies, and are widely used in various fields of pharmaceutical products, cosmetic products, foods and the like. Furthermore, in either of these uses, it is widely known that monodispersed resin particles exert excellent performances.
For example, in solid powder cosmetic products such as makeup cosmetic products, products that are excellent in moldability and impact resistance, are excellent in tight-adhesibility, and have fine extensibility that gives smooth touch but does not give uncomfortable feeling to the skin are demanded. As a method for improving these disadvantages, for the purpose of improvement of fluidity, lubricity and the like, use of various globular resin powders derived from petroleum such as globular polyethylenes, polystyrenes, nylons, polyurethanes and polyacrylics has been proposed (Patent Literature 1). However, these resin powders have problems in corresponding to environments and applying to humans. Therefore, in recent years, polyester-based resin particles derived from plants, such as polylactate, gain attention from the viewpoints of corresponding to environments, affinity to the skin, and the like.
As methods for producing polymer microparticles, in vinyl-based polymers, production of monodispersed min particles has been tried by adding various devises to polymerization methods such as an emulsification polymerization method, a suspension polymerization method, a seed polymerization method and a dispersion polymerization method (Patent Literatures 2 to 4). On the other hand, in polyester-based resins, the polymerization methods thereof are originally different from the above-mentioned polymerization methods.
In the production of polyester-based resin particles, for example, a method including shattering a polylactic acid-based resin in the form of pellets, chips, a bulk or the like by an air jet mill, and a method including shattering at a low temperature by a disc mill at a quite low temperature of liquid nitrogen (hereinafter referred to as a frost shattering method) have been proposed (Patent Literatures 5 and 6). However, the frost shattering method requires equipment for cooling, shattering, classification and the like, and the polylactic acid powder obtained by the shattering method is not globular particles.
Furthermore, a method for obtaining monodispersed resin particles including heat melting, kneading under dispersing, and cooling a resin together with a dispersion medium such as a polyalkylene oxide, a polyalkylenecarboxylic acid or the like in an extrusion molding machine, and dissolving the dispersion medium with a developing solvent such as water (hereinafter referred to as an extrusion molding method) (Patent Literatures 7 to 9) is considered. The extrusion molding method has disadvantages such as decrease in molecular weight and coloring by thermal history; and since the extrusion molding method uses the dispersion medium in a large amount, it is difficult to sufficiently remove the dispersion medium from resin particles, and thus the operations for the removal become complex and expensive. The resin particles obtained by the extrusion molding method are approximately globular, but are difficult to be said to be spherical monodispersed resin particles, and the removal of the dispersion medium is generally insufficient, and thus the surfaces of the resin particles become hydrophilic. Therefore, the resin particles were insufficient for satisfying requirements such as “makeup lasting” in the field of cosmetic products such as powder foundations, for which water resistance is required. Therefore, attainment of spherical monodispersed polyester resin particles having high affinity to the skin, and are excellent in water resistance and thus do not require a hydrophobization treatment has been awaited. Furthermore, in the field of cosmetic products, for example, in the case of use as a scrubbing agent, since resin particles obtained by a shattering method or an extrusion molding method are not spherical, monodispersed resin particles, there are problems that entering of the resin particles into pores and friction on the skin are unstable, and thus the washing of smudge, cuticle and the like is insufficient, uncomfortable feeling is provided to the skin, and the skin is scratched, and the like.
Furthermore, a method for obtaining monodispersed resin particles by dissolving a resin in a solvent, allowing the solution to pass through a porous film, and removing the solvent (Patent Literatures 10 and 11) is proposed. Although a resin dispersion having high monodispersity can be obtained, the production method is complex and requires a special apparatus, and further requires much time and great care. Therefore, the resin particles become extremely expensive.
Furthermore, it is proposed to obtain porous oil-absorbable polylactic acid-based resin microparticles for cosmetic products, or spherical polylactic acid-based resin microparticles having smooth surfaces for toners and the like by forming a two-phase separation system by using an ether-based organic solvent, applying a shear force to this system to give an emulsion, and further bringing the emulsion into contact with a poor solvent having a smaller solubility of the polylactic acid-based resin than that of the ether system to give microparticles (Patent Literature 12), but thus method is not practical in easiness, efficiency, costs and the like from the viewpoints of selective combination of the solvents and the operations and the control thereof, and the monodispersity and stability of the obtained microparticles were not practically satisfiable in the microparticles and the aqueous dispersion thereof.
For example, in the conventional technologies relating to resin particles such as polylactic acid-based particles and aqueous dispersions thereof as mentioned above, resin particles that are spherical, are excellent in monodispersity, and can be produced efficiently at low costs, have not been actually attained.
The applicant also proposed to obtain an aqueous dispersion of biodegradable resin particles by using an ionic polymer as a dispersing agent and by means of stirring (Patent Literature 13), but a room for further improvement and enhancement has been left in stably obtaining spherical monodispersed polyester resin particles and an aqueous dispersion thereof.